Autism is a serious, heritable neurodevelopmental disorder that affects children's functioning in social, emotional, and cognitive domains. Early diagnosis and intervention may ameliorate problems in some children, but autism is difficult to diagnose before age 3. This is because early signs are often subtle; in typical children, development is often uneven; and milder disorders and specific language delays also may be confused with autism in early childhood. Prospective studies of younger siblings of children with an autism diagnosis hold promise, given their higher genetic risk to develop autism or a milder disorder on the autism spectrum (ASD), with recurrence rates as high as 20%. Younger siblings of children with autism (high risk) and age/sex-matched younger siblings of typically developing children (low risk) are currently being studied as part of the Pitt Early Autism Study. The aim of this proposal is to: 1) identify differences between high and low risk toddlers in social-emotional development, focusing on several fundamental aspects of typical development across 18-36 months, that are also known to be impaired in preschool age children with an autism diagnosis, and are especially likely to be disrupted in high risk toddlers who show atypical development; 2) delineate longitudinal trajectories of social-emotional development in high risk toddlers who receive a diagnosis of autism by 36 months and in those who show milder ASD symptoms or language delays, in comparison to high risk toddlers who develop without evident problems and to low risk toddlers; 3) examine profiles of social-emotional, cognitive, communicative, and attentional development in high risk children with and without a later diagnosis and in low risk children. Early social-emotional reciprocity with primary caregivers, the emergence of empathy, the development of pretend play and self-other representation, and the regulation of behavior and negative emotion, salient developmental tasks of toddlerhood, will be assessed. Cross- sectional and longitudinal analyses will delineate group differences and trajectories of typical and atypical development as a function of diagnosis at 36 months. Findings have implications for the early identification and treatment of children with autism and will contribute to differential diagnosis of autism, ASD, and language delay as distinct from typical, but uneven development.